What is your favorite Skill so far in terms of Design, Specialization and interaction with other skills?

I nominate Summon Wolf. Specialization really makes Wolves strong, really strong. The Bleeding route with Beastmaster is excellent and the Lightning route with Shaman is even more amazing IMO. The specialization allows you to build tougher, or faster or wolves that Leap (although the Leap skill is less important when Wolves are running at full speed).

I even tried the Rapid Summoning route which is kind of interesting.

Overall, Wolves are very strong and even more so with The Fang amulet. You can’t build a bad wolf. lol

I haven’t played every class variation yet, have 7 characters so far, so I didn’t try out every skill available, but on one of my playthroughs found the Calamity helmet and the Eye of Reen (both boost fire damage) and figured: let’s try a spellblade mage. I levelled up every fireskill that there is to see which is most useful for me personally and fun to play and look at. And as it turned out it’s Fireball (which I found kinda dull in the beginning), with the flamethrower specialization. It does a massive amount of damage, is fun to play and it looks pretty great to boot. Also love Tempest, my main in D3 was whirlwind barbarian (I think I have over 1200 hours on him alone, though I tested a lot of builds over the years), so it’s no surprise. It also feels great to use it in this game, tried the demo before I bought the supporter pack, saw that skill, played around with it and almost instantly knew I had to get into the alpha. I think it must be very hard to get that feeling down, that these spin attacks feel like they have actual weight behind it and so on, and I think they did a very good job with it.
So these are my favorites so far. And agree with Summon Wolf with The Fang amulet, it’s a very strong combination.

Focusing on necro for right now I have to say it is a toss up of summon skeletons (archers, just got to the arena and archers with multi in the early waves are tearing through everything with terrible gear) and rip blood. I am running a combo minion and rip blood build, going into the mana and damage increase per minion nodes for rip blood.

Ive only played acolyte and sentinel to higher levels, but Id say that so far my favorite skill is tempest with the Tempestual Grasp specialization. With a lot of block and hammer throw to regain mana its pretty safe and effective.